1. Field of the Invention
The present invention relates to processes for the formation of sintered ceramic materials.
2. Description of the Prior Art
Ceramic materials find wide and diverse use in the fabrication of various types of materials. Fairly recently, ceramic materials have found extensive use in the fabrication of electrical components, the ceramic materials typically comprising a ceramic substrate upon which is applied various conductive paths known as metallurgy, to which various connections are joined by appropriate techniques.
While many techniques are known for forming ceramic substrates for use in fabricating electrical components, one of the most popular procedures for fabricating ceramic components involves the casting of what is termed a "ceramic green sheet" and the subsequent firing of the ceramic green sheet.
In greater detail, the casting operation, typically a doctor blade casting, involves forming a casting composition or slip which typically comprises a ceramic frit with an optional glass frit, a binder, typically a thermoplastic polymer, a solvent for the binder to permit the same to be dissolved to form the ceramic slip, and optionally a wetting agent for the system, which permits easy casting of a homogenous slip.
Generally, the ceramic and, if present, glass frit, are classified to the desired size, and then mixed with a solution of the binder in the solvent with optional wetting agent. Mixing is typically in a ball mill for an extended period of time to prepare the casting composition. Hereafter, for purposes of brevity, the term "ceramic" will be used to include such ceramic-glass systems are are commonly used in the art.
The casting composition is then generally cast or spread onto a carrier using the well known doctor blade technique and thereafter the casting composition solvent, or casting composition solvent system where a plurality of solvents are used, is subsequently volatilized to provide a coherent and self-supporting flexible "ceramic green sheet".
The ceramic green sheet is then subjected to various processing operations as are known in the art, for example, punching, screening and the like, and eventually fired to decompose the binder into gaseous combustion products and sinter the ceramic, and if present, glass frit, into a densified ceramic substrate.
After firing, genrally an appropriate metallurgy is formed on the ceramic substrate to permit interconnection with various other electrical components as may be desired.
Recently, such ceramic substrates have found particular use in the fabrication of multilevel ceramic circuit structures, and have achieved extensive acceptance in the electronics industry for the packaging of semi-conductors into integrated devices and other elements as for example, in U.S. Pat. Nos. 3,379,943 Breedlove and 3,502,520 Schwartz.
In the computer industry, ceramic substrates have recently been utilized to form materials known as multilayer ceramic modules, commonly abbreviated MLC in the art. Such MLC's are typically comprised of a plurality of layers of ceramic insulating material (ceramic substrates) separated by appropriate conductive metallurgy to provide internal component interconnections and provided with appropriate conductive surface metallurgy to enable interconnection with other components. The individual ceramic layers in an MLC can be formed of the same ceramic material or of different ceramic materials.
Such multilayer ceramic structures and similar materials are described in, for example, "Laminated Ceramics", Schwartz et al., Ceramic Age, June 1967, pages 40 - 44; "Ceramics for Packaging", Wilcox, Solid State Technology, Parts 1 and 2 in January and February, 1971 issues, beginning at pages 40 and 55; "A Fabrication Technique for Multi-Layer Ceramic Modules", Kaiser et al., Solid State Technology, May 1972, page 35; "Metal-Ceramic Constraints for Multi-Layer Electronic Packages", Chance et al., Proceedings of the IEEE, 59, 1455 (1971); R. D. McBrayer, "Glass-Ceramic -- A New Multilayer Substrate Technology," Presented at the 73rd Annual Meeting of the American Ceramic Society, Chicago, Ill., April 1971; K. Ettre and G. R. Castles, "Pressure Fusible Tapes for Multilayer Structures," Bull. Amer. Ceram. Soc. 51, 482 (1972); R. G. Loasby, "Aspects of Screen Printed Multilayer Networkds," Presented at the 73rd Annual Meeting of the American Ceramic Society, Chicago, Ill., April 1971; F. J. Bachner and H. H. Pichler, "Multilayer Interconnections on Ceramic," ibid.
MLC fabrication is similar to the general ceramic substrate fabrication described above, but generally the ceramic green sheet is laminated to other green sheets of a similar or different composition to form the composite module or final monolithic device. For example, generally a plurality of green cast sheets are blanked into an appropriate size and reference holes punched therein for accurate stacking and orientation. Mechanical punching is typically used. The thus punched blanks are stacked and then laminated at elevated temperature and pressure to firmly adhere the individual green ceramic sheets to each other.
The "green composite" thus formed is generally blanked to nominal device dimensions, allowing for linear shrinkage during sintering, placed on a refractory plate and sintered at elevated temperature in the requisite atmosphere for the ceramic involved.
As earlier indicated, the green ceramic sheet comprises, as major components, the ceramic and a binder. During temperature elevation to the final sintering temperature the binder is burned off or combusted. At the time of final sintering, generally only the ceramic and metallurgy, if a co-sintering is conducted, will be present.
In fabricating MLC materials as described above, it is absolutely essential that the green ceramic sheet illustrate dimensional stability prior to firing, as MLC's are subject to extremely exact device tolerances. In fact, even a slight variation from the design requirements of the final MLC component can lead to device rejection or device inoperability.
If green ceramic sheets can be used substantially immediately after casting and casting composition solvent or solvent system removal, little opportunity exists for changes in dimensional tolerances of the ceramic green sheet. However, I have discovered that if the ceramic green sheet cannot, for some reason, be used immediately after casting composition solvent or solvent system removal, dimensional changes occur in the green ceramic sheet due to stresses developed subsequent to ceramic green sheet formation, which stresses generally cause, with the passage of time, unacceptable changes in the dimensional tolerances of the green ceramic sheet, rendering the same unusable for electronic component fabrication, such as MLC fabrication, where critical device tolerances are mandatory.